Topic overview

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Benign tumors generally remain localized, are often well-differentiated and encapsulated, and may grow slowly or produce normal substances in excess. Malignant tumors can invade surrounding tissues and may spread by lymphatic routes, through the bloodstream, or by seeding. Metastasis is the most reliable hallmark of malignancy, while its absence does not by itself establish benignity. Other features that help distinguish the groups include mitotic activity, anaplasia, necrosis, hemorrhage, functional changes, and host effects. Tumor names also reflect tissue of origin: carcinomas arise from epithelium, whereas sarcomas arise from mesenchyme. These characteristics provide a framework for comparing tumors without treating any single feature other than metastatic spread as definitive.